This invention relates to a device for optical scanning of a field of vision divided into different zones, and for the visual display of the aforesaid field. The invention concerns more particularly a device performing field scanning in two directions according to beams from the aforesaid different zones, and which converge on an element sensitive to the radiation contained in the aforesaid beams, the scanner being used optically regardless of the wavelength of the radiation.
Devices of this type are used in particular in terrain visual display equipment in which the terrain is seen according to infrared radiation with a wavelength greater than one micron (1.mu.). A device such as this, in which the objective is a spherical mirror, and scanning takes place uniquely with mirrors, has been described in French Pat. No. 1,494,805. This known device is of interest because it can be used with a substantial field of vision which, in the limit, could be 180.degree.. On the other hand, it is not well adapted to analyzing a small field with high resolution, which is the case when it is necessary to detect and recognize distant objects in the field of vision, and seen at a reduced angle. It thus is necessary, in effect, to capture a sufficient quantity of the radiation flux emitted by these distant objects. This flux is a function of the surface of the entrance aperture of the device, and actually, in this patent, of the radius of the circle of analysis, and of the numerical aperture of the convergence optics, such that the focal length of the objective is, in principle, equal to the aforesaid radius of analysis.
One possibility of increasing the quantity of flux captured would be to enlarge the aperture of the optics used to converge the beams at the detector. But as this is repeated in several patterns arranged alongside each other about one of the axes of rotation of the scanning system, the number of patterns would be fewer, and this in turn would lead to an increase in the rate of rotation of the aforesaid convergence optics for the same scanning rate, and would pose mechanical realization problems that would be difficult to resolve. Another possibility would be to increase all dimensions of the device according to similarity in construction and position. This, however, would involve excessive bulkiness and too high a moment of inertia of the rotating part.
French patent request No. 73 28 824, filed in the name of the applicant on 7 August 1973, disclosed remedies for some of the above-cited disadvantages. What is realized in order to arrange it so the diameter of the entrance aperture of the device be maximum, and the field of analysis be small, is a system in which the focal length is not bound to the radius of the circle of analysis. This has been done by introducing an optical lens or mirror into the plane of the objective, the purpose of which is to optically conjugate the center of the exit aperture of the objective with that of the detector placed at the center of rotation of the analysis system, the aforesaid element having the advantage of being able to correct the curvature of field of the objective analysis device assembly.
In the device in accordance with this application for a patent, field scanning in one of the directions is accomplished by oscillating the mirror objective about one of its diameters. One of the difficulties that must be resolved with respect to the aperture, when this objective has a large diameter, is how to oscillate a heavy objective at a frequency which, for television images, is of the order of 20 periods per second. Moreover, this mirror objective, even if parabolic, has a field that is limited to few degrees for different aberrations.
In addition, the field of view of the detector must be large enough to accept the flux that traverses the rotating optics during their rotation. The detector, therefore, sees the interior of the analysis device holding the optics, and which device has, most of the time, because of friction, a temperature higher than that of the terrain analyzed. This, in the case of infrared analysis, is followed by the appearance of parasitic modulation of the flux falling on the detector in phase with the line scanning.
Moreover, there is little possibility of placing the detector and the electroluminescent diode in the visible region of the spectrum with a view of direct visual display of the field image by using the analysis system with the reciprocal trajectory because the field image is not isolated.
Then again, in this application for a patent the optics, because they are not uniquely composed of mirrors, make this visual display possible only if the optics are transparent to visible light, something that is difficult to realize when the analysis is made in the far infrared (10.mu.).